To produce useful power from nuclear processes it is necessary to assemble a sufficient concentration of fissionable uranium, or other suitable material, in a physical configuration that will sustain a continuous sequence of energy-producing reactions. This assembly, or reactor core, transfers the heat that is generated in the fission reactions to a working fluid. Frequently, pressurized water flowing through the core at high velocities is used for this purpose.
Because the heat, vibration and radiation that is generated within a power reactor core creates a generally hostile environment, the structural integrity of the core components is an important consideration. Consequently, pressurized water power reactor cores frequently are comprised of groups of fuel assemblies that are arranged in a generally right circular cylindrical configuration. Each fuel assembly, moreover usually comprises an array of about two hundred long slender fuel rods that are parallel to and spaced from each other. Each of these fuel rods contains a stack of generally cylindrical uranium dioxide pellets in which the uranium provides the fissionable fuel for the power reactor.
These fuel assemblies are not limited to fuel rods but also support a number of other components. For example, instrumentation tubes for observing temperature and neutron flux conditions within the core; end fittings and fuel element grids for stabilizing the fuel assembly components; and control rods and control rod guide tubes for regulating the power output from the reactor through the selective absorption of fission inducing neutrons within the reactor core often are made a part of the fuel assembly structure.
Clearly, the neutron distribution will vary from place-to-place within the reactor core. Illustratively, near the core perimeter it can be expected that the neutron population will be small relative to the center of the reactor core because the concentration of neutron producing uranium is lower at the core perimeter than it is in the center of the core. Neutrons at the core perimeter can "escape" from the core more readily through the core surface than they can from the center of the core, further tending to reduce the neutron concentration near the reactor core surface. Because heat generation within any specific portion of the reactor core is related to the neutron population within that portion, there is a definite tendency to produce higher temperatures at the center of the reactor core than at its margin. This inclination toward producing local temperature maxima in different regions within the reactor core is generally undesirable for a number of reasons. Primarily, the reactor is designed for core operation that will not exceed a predetermined temperature. If this core temperature is reached in just one or in a few local points within the reactor core, the over-all heat generating potential of the core can not be realized. This effect results because the temperatures elsewhere in the core must be kept to lower values in order not to exceed the design temperature at those limited points or "hot spots" in which the maximum design temperature has been reached.
Accordingly, in the simplified example under consideration, over-all reactor power can be increased if the neutron population (and hence, heat) in the central portion of the core is depressed and the neutron population in the larger volume that characterizes the peripheral reactor core annulus is allowed to increase. In this way, by "flattening" the power distribution in the reactor core, the core actually is able to generate more power than it would be able to generate if the neutron concentration, temperature and power was allowed to reach a peak in the center of the core, or in some other location, as the case may be. In order to achieve this "flat" power distribution, it has been the practice to insert "burnable poison" rods in the fuel assemblies. Typically, a burnable poison rod is a tube filled with a material that has a very high probability for absorbing neutrons. For example, a sintered dispersion of boron carbide in an alumina matrix is suitable for this purpose.
Neutrons, absorbed in this manner by means of the material within the rod are, in effect, withdrawn from the fission and power generation process. And so, to "flatten" the power distribution with a reactor core, burnable poison rods are concentrated in those fuel assemblies that are located in the central portion of the reactor core.
Depending on a number of subtle effects it also might be advisable to provide burnable poison rod concentrations in other portions of the reactor core in which specific design or operational features produce large local neutron populations.
Not only must the fuel assembly support all of these diverse structural components in spite of the generally hostile environment within a reactor core, but the fuel assembly also must be capable of the somewhat conflicting need for swift and easy disassembly. For instance, it should be noted that fuel assemblies become radioactive after exposure in an operating reactor core. This radiation is so intense that inspection and repair can be accomplished only with remote handling equipment behind adequate radiation shielding.
Consequently, because disassembly procedures are expensive and time consuming, the need for a sturdy, yet readily dismountable structure is of considerable commercial importance.
The burnable poison rods that are used in many fuel assemblies are a part of this structural picture. Generally, the burnable poison rods that have characterized the prior art are mounted for movement in a direction that is parallel to the longitudinal orientation of the fuel rods. A "spider," in the form of a centrally disposed hub from which a number of arms radiate often is used to couple the burnable poison rods together for longitudinal movement relative to the balance of the fuel assembly. This motion is required to permit the power reactor operator to insert or withdraw the burnable poison rods from the reactor core in response to power flattening needs.
Eventually, lumped burnable poison rods must be removed from operation, packaged in a cask that provides adequate radiation shielding and then shipped for disposal at a suitable site. It is desirable to remove each of the rods from the spider in order to economize on the volume of the shipping cask. Unfortunately, removing the burnable poison rods from the spider is complicated and potentially hazardous for a number of reasons. The irradiated poison rods have developed, after sufficient irradiation in the reactor core, an internal gas pressure. The cladding or tubing in which the burnable poison has been loaded also becomes quite brittle as a result of a period of irradiation. The burnable poison rods usually are joined to their respective spider arms by means of threaded fasteners. In these circumstances, the most frequently suggested techniques for removing the rods from the spider are by means of shearing or sawing. Sawing the rods permits these rods to be handled more gently--an important consideration in view of the gas pressure within the rods--but the sawing process generates radioactive chips. Shearing overcomes this problem to a large extent, but does nevertheless result in undesirably rough handling.
There is, then a need for some suitable means for joining burnable poison rods to the spider arms in a manner that is proof against the reactor core environment but permits these rods to be removed from the spider simply, swiftly, and delicately.